The present disclosure relates to an image forming apparatus which forms an image on a recording sheet. More particularly, the present disclosure relates to a method for controlling a charging bias for electrostatically charging an image carrier on the surface of which a toner image is formed.
Among image forming apparatuses such as printers, copiers, facsimile machines, multifunction peripherals having the functions of more than one of them, etc., some are known which are provided with a photosensitive drum as an example of an electrophotographic photosensitive member, a charging member such as a charging roller which electrostatically charges the surface of the photosensitive drum, and a cleaning blade which is arranged in contact with the surface of the photosensitive drum and which removes toner or external additive left unused on the surface of the photosensitive drum.
Such a charging member is arranged in contact with or close to the image carrier, and discharge products produced due to electric discharge by the charging member attach to the surface of the image carrier. This increases the friction resistance between the surface of the image carrier and the cleaning blade, and makes the cleaning blade more likely to suffer from chatter, tears, and stick-slip, resulting in degraded cleaning performance of the cleaning blade. As a result, with an increased slipping amount of toner and external additive, the charging member is contaminated, and toner and external additive left uncleaned are fixed to the surface of the image carrier, resulting in image quality degradation and image formation defects.
In particular, when an image carrier having on its surface an amorphous silicon layer formed as a photosensitive layer is used, at an early stage after the start of use, owing to surface irregularities ascribable to crystal particles produced when the amorphous silicon layer is formed, the contact area is small between the surface of the image carrier and the cleaning blade, and thus the friction resistance between them is also small; however, as the image carrier continues being used, the irregularities on the surface of the image carrier wear and smoothen, with the result that the friction resistance increases between the surface of the image carrier and the cleaning blade, making the previously mentioned problems more likely to occur.
As a solution, a configuration is known in which an amorphous silicon layer is formed on an aluminum tube having a large surface roughness so as to give the amorphous silicon layer a large surface roughness in advance with a view to suppressing an increase in the friction resistance and degradation in cleaning performance of the cleaning blade. Moreover, another configuration is known in which the driving electric current for the image carrier is monitored and, when the value of the driving electric current is larger than a predetermined value, the AC voltage applied to the charging device is reduced, with a view to suppressing production of discharge products.